Non-linear X-ray optics: Stimulated Raman Scattering in the Impulsive Limit

Stimulated x-ray Raman scattering from liquid water using attosecond pulses has been observed for the first time.

The study, reported in Science Advances today and led by Imperial College London researchers, observed stimulated x-ray Raman scattering from liquid water using attosecond pulses at the LCLS x-ray free electron laser.

Raman scattering is a class of two-photon non-linear optical process in which a photon is absorbed by a medium at one frequency, and another photon emitted at a different frequency (the Stokes frequency), with the energy difference being accounted for by excitation of the medium.

This has led to the Raman spectroscopy technique that is widely used in the optical and infra-red wavelength range to probe the vibrational excitation spectrum of molecules and materials. With x-rays, Raman scattering can lead to excitation of electronic states of a material.

In contrast to previous measurements with spontaneous x-ray Raman scattering and stimulated x-ray Raman scattering with stochastic multimode x-rays, in this new study the team used single pulses of ~ 400 as duration with a coherent energy bandwidth of > 7 eV to excite a superposition of electronic states in the water molecule. The excited state spectrum is encoded into the Raman emission photon spectrum.

Lead researcher Professor Jon Marangos, from the Department of Physics at Imperial, said: “The importance of this result is that it extends the study of impulsively excited electronic states to materials in the condensed phase and advances a general spectroscopic method for measuring electron quantum superpositions on ultrafast timescales in neutral matter.”

First author Dr Oliver Alexander, also from the Department of Physics at Imperial, added: “This opens the door to a broad class of previously inaccessible measurements involving coherent valence electronic motion in neutral molecules with attosecond temporal and Angstrom spatial resolution. This creates new opportunities for the attosecond time domain probing of the electronic dynamics and couplings in materials, chemistry, photobiology and solar light harvesting systems.”

Moreover, whereas the current study was restricted to 120 Hz repetition rate and with other instrumentation limitations, new machines (like LCLS II) are now commissioned that with dedicated instrumentation will increase the data rate by factors > 1000 and so enable measurements of weak time-resolved stimulated Raman emission from complex materials in a few seconds rather than the hours of data taking in this study.

'Attosecond Impulsive Stimulated X-ray Raman Scattering in Liquid Water' by Oliver Alexander et al. is published in Science Advances.